A coating composition for coating the top surface of an automobile, generally contains, as a film forming binder, a combination of a hydroxyl group-containing polymer and a melamine curing agent. The cured film obtained from the melamine curing system, however, has poor acid resistance and is damaged by acid rain, which has recently become one of serious problems. The damage of the film results in poor appearance.
The poor acid resistance which is provided by using a melamine resin is believed to be originate from a triazine nucleus in the melamine resin. Accordingly, a drawback of poor acid resistance is not dissolved, so far as a melamine resin is used as a curing agent.
A novel coating compositions which does not employ the melamine curing agent is proposed in Japanese Kokai Publications 45577/1990, 287650/1991. The similar coating compositions are also disclosed in U.S. Pat. Nos. 2,977,334 (to Zopf et al.), 3,399,109 (to Zimmerman et al.) and 3,528,935 (to Marion et al.). The proposed coating compositions are acid-epoxy type, and an acid group and an epoxy group contained therein reacts to form an ester bond when curing. The ester bond functions as a crosslinking point of the coated film.
The coated film which has a crosslinking point of ester bond has good acid resistance and sufficient weather resistance for an automotive top coating application, by comparison with the conventional acrylic-melamine type coated film.
However, the coated film h s a problem that it is easily damaged by brushing, for example, in the use of a car washing machine. If degree of crosslinking is raised in order to impart good mar resistance against the mechanical car washing in this curing system, the resulting cured film has poor extensibility and becomes hard and brittle. The hard and brittle film brings another problems that it easily cracks, particularly when it is formed on a sealer for an automobile which is a resin material for filling in a gap between sheet metals, because such a film hardly responds to expansion and contraction of the sealer. It is therefore difficult to use the proposed coating compositions as a top coating composition for the automobiles.
In order to solve the above mentioned problems, the applicants proposed a coating composition which includes an acid component having a soft segment. The resulting coating composition however has poor curability and storage stability, and it should also be more improved in mar resistance.